This application is based upon and claims priority of Japanese Patent Application No. 11-75961 filed Mar. 19, 1999, the contents being incorporated herein by reference.
1. Field of the Invention
The present invention relates to a magnetic recording medium and, more specifically, it relates to a magnetic recording medium that can be advantageously used for hard disk drive (hereinafter, referred to as xe2x80x9cHDDxe2x80x9d) devices in computers. The present invention also relates to a process for the production of the magnetic recording medium.
2. Description of the Prior Art
As is well-known, magnetic recording media are constructed with a non-magnetic base and a magnetic layer, i.e. a thin film of a magnetic recording material, formed on the base, and an underlayer is usually formed between the non-magnetic base and the magnetic layer in order to control the crystal orientation of the magnetic particles composing the magnetic layer. More specifically, the most commonly used form of magnetic recording medium at the current time has an underlayer of chromium (Cr), titanium (Ti) or an alloy composed mainly thereof formed on a non-magnetic base such as a glass or silicon plate, and a magnetic layer made of a cobaltchromium (hereinafter, xe2x80x9cCoCrxe2x80x9d)-based alloy, composed mainly of Co, formed on the underlayer.
Incidentally, with the remarkable increase in HDD capacities in recent years, demands have been increasing for improved recording densities, or higher recording densities, in the magnetic recording media used therein as well. Realization of such higher recording density has required the magnetic layers in the magnetic recording media to be designed with smaller thicknesses, higher resolution, greater magnetic coercive force and lower noise. In other words, the higher density of magnetic recording media has resulted in a reduction in the area of the magnetic layer occupied per bit. Because of this situation, the leakage magnetic field generated from the magnetized region of each bit can only be guaranteed by reducing the thickness of the magnetic layer in conformity with the reduced bit size, to ensure a semi-circular arcuate magnetic field state and minimize loss of signal output. Furthermore, because the bit spacings must be narrow, improvement in the magnetic domain structure in the magnetized transition region to reduce noise requires fine crystalline particles, to match the lower thickness, and reduced interaction between magnetic particles. Such noise reduction makes it possible to achieve both higher resolution and higher magnetic coercive force.
In conventionally formed magnetic recording media, Cr is segregated at the crystalline grain boundaries in a CoCr-based alloy polycrystalline film constituting the magnetic layer, and this region is demagnetized to attempt to reduce the interaction between particles. The method of segregating the Cr may be, for example, a method whereby a magnetic layer with a polycrystalline structure is deposited by sputtering on a heated base, and the Cr in the target is segregated at the crystalline grain boundaries during the deposition. Here, the Cr segregation is often promoted by increasing the proportion of Cr added to the magnetic layer alloy, by adding other elements such as Ta that can effectively promote Cr diffusion, or by heating the base at high temperature during deposition of the magnetic layer. A typical temperature for heating of the base is 200-300xc2x0 C.
However, while employment of such means that promote Cr segregation can provide an effect of promoting Cr segregation and thereby reducing interaction between particles, it also has an adverse effect on fine crystal grain formation which is one of the objects, and therefore it is difficult to achieve and control both simultaneously. In fact, even when the magnetic layer thickness is reduced to a film thickness of 20 nm or less which is necessary to achieve high recording densities of 10 Gb/in2 and greater, it is difficult to realize adequate segregation of Cr with fine crystal grain formation to a planar crystal grain size of 20 nm or less. Also, heating of the base during deposition releases adsorption gas present in the base and the film forming chamber, thus undesirably reducing the degree of vacuum in the chamber, while the gas discharge volume and the type of gas also change depending on the humidity and the state of the adhesion film in the chamber, thus undesirably causing instability in the properties of the formed magnetic layer.
It is an object of the present invention to overcome the aforementioned problems of the prior art by providing a magnetic recording medium that can be advantageously used, for HDDs in particular, and that can realize high recording density.
It is another object of the invention to provide an advantageous process for production of such a magnetic recording medium.
These and other objects of the invention will become apparent in the detailed description which follows.
According to one aspect of the invention, there is provided a magnletic recording medium comprising a non-maginetic base and a non-magnetic underlayer and a magnetic layer formed in that order on the base, the magnetic recording medium being characterized in that
the underlayer comprises a nonmagnetic polycrystalline material, and
the magnetic layer comprises magnetic cobaltchromium (CoCr)-based alloy particles, wherein the planar size of the magnetic alloy particles is defined by the planar size of the polycrystalline particles comprising the underlayer, and is no greater than 20 nm.
According to another aspect of the invention, there is also provided a process for production of a magnetic recording medium comprising a nonmagnetic base and a non-magnetic underlayer and a magnetic layer formed in that order on the base, the process for production of a magnetic recording medium being characterized by
forming the underlayer by depositing by magnetic polycrystallinie material on the non-magnietic base while the base is in a vacuum under non-heated conditions, and
forming the magnetic layer by depositing magnetic CoCr-based alloy particles on the underlayer while the base is in a vacuum under non-heated conditions, wherein the planar size of the magnetic alloy particles is defined by the planar size of the polycrystalline particles composing the uniderlayer, and is no greater than 20 nm.
According to the invention, the aforementioned problems may be solved by employing means whereby a laminated structure is realized wherein the planar size of the crystal grains composing the underlayer determines the planar size of the crystal grains composing the magnetic layer deposited on the upper layer, to control the planar magnetic particle size. Also, by promoting segregation of the non-magnetic metal element at the crystal grain boundaries of the magnetic layer with a polycrystalline structure (hereunder, this will also be referred to as xe2x80x9cCr segregationxe2x80x9d, for Cr as a typical non-magnetic metal) without altering the laminated structure, it becomes possible to reduce interaction between the particles and thus independently control the Cr segregation and the magnetic particle size. According to the invention, it is useful to employ a medium-forming technique wherein the laminated structure and promoted Cr segregation are accomplished by deposition of the underlayer and magnetic layer on a non-heated base (room temperature deposition; preferably using sputtering as the deposition method), to form the desired laminated structure, followed by post-annealing to promote Cr segregation. In this medium-forming technique, the Cr segregation can be easily controlled by controlling the post-annealing temperature. Control of the planar crystal grain size of the underlayer, which determines the planar size of the magnetic-particles, can be easily accomplished by changing the film thickness and deposition rate of the underlayer. Specifically, it may be accomplished by reducing the thickness of the underlayer or lowering the deposition rate to 5 nm/sec or lower.
The present inventors have discovered a phenomenon whereby, when the planar size of the magnetic particles is 20 nm or smaller and the planar size of the magnetic particles alone is reduced to a fine size while the width of the non-magnetic region at the crystal grain boundaries which is formed by Cr segregation remains the same, the interaction between particles tends to increase, lowering the magnetic coercive force. The present invention, however, attempts to reduce medium noise while increasing the interaction between particles. The aforementioned phenomenon would therefore not be an issue if the problem of lower magnetic coercive force reduction can be solved. The present inventors have found that the problem of lower magnetic coercive force can be solved by adding a higher proportion of Pt to the CoCr-based alloy.